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If we do the calculation for a short peptide of 10 amino acids, we arrive at an enormous 10,240,000,000,000 combinations. It becomes apparent that even a dipeptide made of just two amino acids joined together gives us 20 2 = 400 different combinations.

To get a sense for the diversity of proteins that can be made using 20 different amino acids, consider that the number of different combinations possible with 20 amino acids is 20 n, where n=the number of amino acids in the chain. Proteins are made by linking together amino acids, with each protein having a characteristic and unique amino acid sequence. The versatility of proteins arises because of their varied structures. Life, as we know it, would not exist if there were no proteins. Among other things, protein enzymes catalyze the vast majority of cellular reactions, mediate signaling, give structure both to cells and to multicellular organisms, and exert control over the expression of genes. Virtually everything that goes on inside of cells happens as a result of the actions of proteins. The entire textbook is available for free from the authors at

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